Golf balls are typically classified into thread wound golf balls and solid golf balls. The solid golf ball is generally composed of a core and a cover covering the core. Although the core can have either single layer structure or multi-layer structure, the core is generally formed from vulcanized rubber. The vulcanized rubber is obtained by vulcanizing or press-molding a rubber composition, which typically comprises high-cis polybutadiene rubber, zinc salt of (meth)acrylic acid, organic peroxide as a crosslinking agent and the other additives. The zinc salt of (meth) acrylic acid acts as a co-crosslinking agent to form crosslinkage through the zinc salt of (meth) acrylic acid between rubber molecules, in addition to crosslinkage through oxygen atoms generated by peroxide between rubber molecules. These two types of crosslinkage enhance flight performance and improve shot feel at the time of hitting of the resulting golf ball. The term “(meth) acrylic acid” as used herein refers to acrylic acid or methacrylic acid, or the mixture thereof.
It has been general that both core and cover have single layer structure, but it is now proposed that the core or cover are made multi-layer structure, which has two or more layers, in order to extend flight distance and improve shot feel. It is, however, difficult to make the core multi-layer structure, particularly two layer structure composed of a center and a core outer layer, because it is required to cover a rubber composition having high viscosity in concentric circle on the center and vulcanize or press-mold it. Methods of producing a two-layered core include, for example,
(A) a method comprising the steps of vulcanizing or press-molding a rubber composition for center to obtain a center, holding the center to a given position in a mold for a core outer layer with a movable hold pin, removing the pin, and then vulcanizing or press-molding the core outer layer,
(B) a method comprising the steps of semi-vulcanizing a rubber composition for core outer layer or heating the rubber composition for a given time to such a degree that the rubber composition does not shrink using a mold having a semi-spherical cavity and a male plug mold to form a semi-vulcanized or unvulcanized semi-spherical half-shell for the core outer layer, removing the male plug mold, putting the vulcanized center in the semi-spherical half-shell for the core outer layer, covering the center with two half-shells, and then vulcanizing or press-molding it,
(C) a method described in the method (B) except that the core outer layer is used as a seat without making the core outer layer the half-shell.
The above are described in Japanese Patent Kokai Publication Nos. 105774/1988, 228978/1990, 218077/1994 and the like. The methods also include;
(D) a method described in the method (B) except that the unvulcanized center is covered with two semi-vulcanized or vulcanized semi-spherical half-shells for core outer layer to vulcanize or integrally press-mold it without vulcanizing the center in advance,
(E) a method comprising the steps of covering the molded center with an unvulcanized core outer layer, and molding it in a mold having a different shape from the mold for vulcanizing or press-molding the core outer layer in a desired shape when forming the core outer layer, and
(F) a method comprising the steps of covering the center with the core outer layer in two stages, comprising the first stage of forming an intermediate article having a Rugby ball shape using a mold having an ellipse cavity having a shorter diameter slightly larger than the outer diameter of the center and a longer diameter equal to the diameter of the core outer layer, and the second stage of putting the longer diameter of the intermediate article on the seam surface in a mold for core to cover the intermediate article with the core outer layer. This method (F) is based on the fact that the eccentricity of the center in the core mainly occurs at the seam surface of the core outer layer.
In the above methods, it is required to use a great quantity of fluororesin type or silicone resin type release agent, or to carry out a special release treatment on a mold surface, such as chrome plating or fluororesin coating, because the releasability of the molded article from the mold is very poor when molding a rubber composition containing zinc acrylate and high-cis polybutadiene.
In case of using the release agent, the release agent usually remains on the surface of the molded article and the adhesion between the center and the core outer layer or that between the core outer layer and the cover is degraded by the presence of the remaining release agent. Accordingly, it is required to remove the release agent by means of buffing or washing. The removal of the release agent remaining in a concave portion of the molded article is very difficult. The special release treatment, such as chrome plating or fluororesin coating is better because the release agent is not adhered to the molded article. The special treatment, however, has a defect that operation is complicated and requires high cost. There has been proposed a release agent that is not adhered to the molded article, but the coating operation is complicated and it is expensive.